Protective treatment for magnesium



lilo-74 United States Patent 3,447,973 PROTECTIVE TREATMENT FOR MAGNESIUM Herbert K. De Long and Donald E. Ritzema, Midland,

Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Aug. 17, 1966, Ser. No. 572,909 Int. Cl. 1323c 3/04, N10 US. Cl. 148--6.16 7 Claims This invention relates to a method for providing a protective surface coating on magnesium and its alloys and more particularly is concerned with a novel process for providing a coating on magnesium articles which is highly resistant to corrosion and which also is adapted for use as a base coating or undercoating for thermoplastic films or sheets, paints, lacquers, enamels and the like surface coatings adhered thereto.

The term magnesium as used herein refers to magnesium metal or a magnesium base alloy which contains about 70 percent or more of magnesium and includes the base metals and their wrought and cast alloy products.

It is a principal object of the present invention to provide a novel process for coating magnesium with a protective surface which provides high resistance to the magnesium surface against corrosive attack by aqueous saline solutions, organic liquids, and the atmosphere.

It is a further object of the present invention to provide a novel, economical method for upgrading the corrosion resistance of magnesium and its alloys, particularly the less corrosion resistant alloys commonly employed in die casting applications.

It is also an object of the present invention to provide a surface coating for magnesium which serves both as a corrosion resistant layer on the surface of the magnesium as well as provides a satisfactory undercoating or base material for the application of conventional finish coatings onto the magnesium.

In general, the present novel process comprises cleaning magnesium in a n aqueous alkaline cleaner containing a water-soluble chromate and heating the so-cleaned chromate treated magnesium. Ordinarily, in the practice of the invention the resulting treated magnesium also is posttreatedwithan aqueous silicate coating although good corrosion resistance is realized from the combination of steps employed in the alkaline chromate treatment and baking.

In the actual practice of the invention, a magnesium article is contacted at a temperature of from about 70 F. to about 220 F. for a period of from about 1 to about 60 minutes with an aguenusmaustic sglutimgntaining p g pha eanq fpmf te ipn sourcegmaten'als. The treating solution comprises an alkali metal caustic (e.g. so-

dium hydroxide or potassium hydroxide) of from about to about 300 grams/ liter, expressed as NaOH, from about 1 to about 60 grams per liter alkali metal phosphate, expressed as trisodium phosphate dodecahydrate and an aqueous soluble chromate ion source of from about 1 to about 50 grams per liter, expressed as sodium chromate tetrahydrate (Na CrO -4H O). Following contact with the alkaline chromate solution, the magnesium article is heated at a temperature of from about 300 to about 800 F. for a period of from about 2 minutes to about 8 hours or more. The heating step is carried out in an oxidizing atmosphere, e.g. in air or oxygen, utilizing temperatures and times within the range set forth directly hereinbefore so as to assure conversion of the resultant chromate coating on the magnesium article to an aqueous insoluble state.

Unexpectedly, magnesium articles treated in accordance with this novel process show a marked resistance to attack both by aqueous saline solutions as well as liquid organics such as hydrocarbons, alcohols and the like, and, natural atmosphere conditions.

A post-treatment with an alkali metal silicate can be employed to further increase the corrosion resistance of the magnesium. In this post-treatment, the chromate coated-heat treated magnesium article is contacted with an aqueous alkali metal silicate solution at a temperature of about 50 to about 180 F. and then dried at a temperature of about 70 to about 800 F. for a period of about 2 minutes to 24 hours or more.

Either water soluble sodium silicates or potassium silicates can be used for this post-treatment. Ordinarily, commercially available aqueous sodium silicate solutions having a Na O:SiO ratio (on a molar basis) of from about 0.6 to about 0.25 or potassium silicate solutions having a K,O:Si0 ratio of from about 0.45 to about 0.4 are used. A particularly suitable silicate composition for the post-treatment is a solution of a commercially available aqueous sodium silicate solution comprising the equivalent of about 8.9 parts by weight Na O-28.7 parts by weight SiO,-62.4 parts by Weight H O admixed with additional water in an amount to provide a solution having at a minimum about 25 volume percent of the original sodium silicate solution. If desired, the sodium silicate solution can be used as received, but economics favor dilution with additional water as such diluted solutions have been found to be satisfactory in the practice of the present invention.

Preferably, in carrying out the present novel process a magnesium article is contacted at a temperature of from about 180 to about 210 F. for a period of from about 3 to about 15 minutes with an alkaline chromate solution comprising from about 60 to about grams/liter sodium hydroxide, about 5 to about 20 grams/liter and chromate ion, equivalent to from about 3 to about 10 grams/liter Na- CrO -4H O. After separation of the article from contact with the aqueous alkaline chromate solution, the treated article is rinsed with water (either cold or hot), and then heated in air at a temperature of from about 450 to about 600 F. for a period of from about 5 to about 60 minutes. The magnesium article usually then is cooled to a maximum temperature at least of about 180 F. The cooled article is contacted at a temperature of from about 70 F. to about 110 F. with an aqueous sodium silicate solution consisting of a solution having the equivalent of about 8.9 parts by weight Na O28.7 parts by weight SiO and 62.4 parts by weight H 0 and diluted with additional water to provide on a volume basis a solution containing from about 30 to about 60 percent of the original sodium silicate solution. After application of the silicate coating, the article preferably is force dried by heating in an oven at a temperature of from about 250 to about 600 F. for a period of from about 5 to about 30 minutes.

If desired, an aqueous soluble borate, e.g. alkali metal metaborate, in the range of from about 5 to about 10 grams/liter and/or an aqueous soluble chromate, expressed as sodium chromate, in the range of from about 2.5 to about 7.5 grams/liter can be incorporated into the silicate solution during the post treatment.

Contacting of the magnesium with the alkaline chromate solution and silicate post-treatment can be carried out by immersing the article in a bath of the treating solution by spraying solution on the article, by wiping or brushing an article, roller coating with the solution or by other liquid-solid contact techniques as understood by one skilled in the art.

Aqueous soluble chromate ion source materials particularly suitable for use in the method of the present invention include, for example, the alkali metal and ammonium chromates and dichromates as well as chromic acid (CrO The post-treatment of silicate coating if followed in the practice of the invention can be applied prior to the elevated temperature bake of the chromate treated article. With this alternative procedure, ordinarily the alkaline chromate treated magnesium material is washed, a silicate coating applied and the material heated within the predetermined temperature range for a period as set forth following standard dip coating techniques so as to leave a residual coating of silicate on the metal surface. The silicate solution employed consisted of a commercially available aqueous sodium silicate composition having a nominal composition equivalent to 8.9 Weight percent Na O28.7 weight percent SiO -62.4 weight percent H O. This solution as used for the silicate treatment first was admixed and diluted with an equal volume of water. Also, for this particular study, sodium tetraborate decahydrate (NaB4O7'1OH O) in an amount to provide l0 herernbefore. This serves not only to effect the requisite grams per liter was added to the silicate solution. insolubilization of the chromate coating on the mag- Following the silicate dip coating treatment these samnesium but also provides for drying the coating. ples were dried by heating in an oven for minutes The present novel alkaline chromate-heat treatment at 350 F. process provides a number of distinct and unexpected ad- 15 Other of the alkaline chromate treated and baked die vantages. During the alkaline chromate treatment, organic cast parts were similarly dip coated in a silicate solution soil, dirt and foreign metallic particles that react with following the same procedure as described directly hereand dissolve in the presence of alkali bases such as, for inbefore which contained about 5 grams per liter sodium example, aluminum, zinc, tin and the like are removed dichromate (Na Cr O -2H O). These were dried in an from the magnesium. Such metallic surface inclusions are oven for 15 minutes at 350 F. undesired since these during service of the magnesium As controls an additional number of die cast parts article, e.g. a die casting, could cause localized pitting type of the same type and composition were prepared wherein corrosive attack because of galvanic cell action. This treatthe metal was grit blasted and no subsequent protective ment serves simultaneously to apply a thin, relatively surface treatment was given to the magnesium or wherein water insoluble, chromate type protective coating on the the order of processing steps was changed from that set magnesium surface. The phosphate ion appears to further forth as required and claimed for operability in the aid in the formation of the protective coating as well as present process. act as a softener for the water solvent. Each of the die cast articles was placed in a hori- As indicated hereinbefore, by following the alkali chrozontal position and exposed to a 5 percent sodium chlomate treatment with a controlled heat treatment unex- 3n ride fog in a standard accelerated salt spray fog corrosion pectedly high resistance to corrosive attack in a wide test in accordance with the requirements set forth in variety of environments is realized. ASTM specification Bll7-6l. Test data and corrosion Additional corrosion resistance is imparted to the magresults from this study are presented in the table which nesium by the post-treatment with silicate solution alone follows.

TABLE Corrosion Alkaline Silicate test results chromate post (percent Run No. treatment Bake treatment attack) Remarks 1 Yes Yes No 18 Yes.. Yes..- Yes 3 3 4 Na1Cr:O1-2H=O in silicate solution. Control A 85 Metal surface cleaned by grit blasting (base only. metal). Control B. No... 75 Bake and grit blast. Control C.. Yes 70 Control D Yes 70 Sample baked before alkaline chromate treatment. N8zCl1O1-2H1O in silicate solution. 20 Sample baked before alkaline chromate treatment.

or with the silicate solution which also contains small Example 2 amounts of alkali metal borate and/or chromate ion.

The following examples will serve to further illustrate the present invention but are not meant to limit it thereto.

Example 1 Magnesium die cast parts fabricated from AZ9lC alloy (ASTM designation; nominal composition Al-8.7%, Mn0.20%, Zn-0.7%, Mg-balance) were cleaned by blasting the surface with an aluminum-copper alloy grit (the alloy contained about 14% Cu and was about the same coarseness of blasting sand) so as to remove machining burrs, major surface roughness and to mask metal surface flow lines thereby producing a uniform appearing metal surface. The parts were immersed in an aqueous alkaline chromate solution for about 5 minutes at 180 F. This solution contained about 60 grams/liter sodium hydroxide, 10 grams/liter trisodium phosphate dodecahydrate and 5 grams/liter sodium dichromate dihydrate (Na Cr O -2H O). After removal from the alkaline-chromate solution, the articles were rinsed in water and placed in a baking oven and heated at about 500 C. for about 30 minutes. Samples were then taken and cooled to room temperature.

Other of the so-treated and baked pieces were cooled to approximately 150 F. and dipped into an aqueous silicate solution and slowly withdrawn from the solution Following the same general procedure and treating solutions as described in Example 1, an ASTM designated AZ92CF sand casting (nominal composition about 9 percent Al, 0.15 percent manganese, 2 percent Zn, balance Mg) was cleaned by the aluminum alloy shot blast followed by aqueous alkaline chromate, baked, and given a one-coat sodium silicate post-treatment. Upon subjecting the so-treated casting to the ASTM designated B117- 61 accelerated salt spray fog corrosion test this test specimen was found to have undergone about 35 percent corrosive attack.

As a control, a sample of the same alloy casting was subjected only to the aluminum alloy shot blast treatment and then subjected to the accelerated salt spray fog corrosion test. This control showed about 95 percent corrosive attack by the salt fog.

In a manner similar to that described for the foregoing examples an alkaline chromate cleaned and heat treated magnesium can be post-treated with an aqueous potassium silicate solution having a K OISiO equivalent as set forth herein.

Various modifications can be made in the process of the present invention without departing from the spirit or scope thereof for it is understood that we limit ours selves only as defined in the appended claims.

We claim:

1. A process for providing a protective surface coat ing on magnesium which comprises:

(a) contacting a magnesium article at a temperature of from about 70 F. to about 220 F. for a period of from about 1 to about 60 minutes with an aqueous alkaline solution containing phosphate and chromate ion source materials, said solution comprising an alkali metal caustic, expressed as NaOH, of from about 5 to about 300 grams/liter, alkali metal phosphate, expressed as trisodium phosphate decahydrate, from about 1 to about 60 grams/liter, and, aqueous soluble chromate ion source material, expressed as sodium chromate tetrahydrate, 'from about 1 to about 50 grams/ liter,

(b) separating the so-treated article from contact with said solution, and

(c) heating the so-treated article at a temperature of from about 300 to about 800 F. for a period of from about 2 minutes to about 8 hours or more in an oxidizing atmosphere.

2. The process as defined in claim 1 wherein the magnesium article is contacted at a temperature of from about 180 to about 210 F. for a period of from about 3 to about 15 minutes with an aqueous alkaline chromate solution comprising from about 60 to about 120 grams/ liter sodium hydroxide, from about 5 to about 20 grams/ liter trisodium phosphate decahydrate and chromate ion equivalent to from about 3 to about 10 grams/liter sodium chromate tetrahydrate and including the steps of rinsing the so-treated article after separation of said article from contact with the aqueous alkaline chromate solution and wherein the rinsed article is heated in air at a temperature of from about 450 to about 600 F. for a period of from about 5 to about 60 minutes,

3. The process as defined in claim 2 and including the steps of contacting the chromate treated-baked magnesium article with a sodium silicate solution at a temperature of from about ransom-r10" F., said sodium silicate comprising the equivalent of about 8.9 parts by weight Na 28.7 parts by weight SiO, and 62.4 parts by weight H O diluted with additional water to provide on a volume basis a solution containing from about 30 to about 60 percent of the original sodium silicate solut-ion, and, hating the resulting silicate coated article at a temperature of from about 250 to about 600 F. for a period of from about to about 30 minutes.

4. The process as defined in claim 3 wherein said silicate solution contains from about 5 to about 10 grams/ liter aqueous soluble borate, from about 2.5 to about 7.5 grams/liter aqueous soluble chromate and mixtures of said borate and said chromate within the disclosed range, said 'borate being expressed as Na,B O -10H,O and said chromate being expressed as N-a CrO -4H O.

5. The process as defined in claim 1 and including the steps of contacting the resulting chromate coated-heat treated magnesium article with an aqueous alkali metal silicate solution at a temperature of from about to about 180 F., said silicate solution being a member selected from the group consisting of sodium silicate and potassium silicate, said sodium silicate solution further being characterized as having a Na OzSio molar ratio of from about 0.6 to about 0.25, and said potassium silicate solution being further characterized as having a K O:SiO, molar ratio of from about 0.45 to about 0.4, and heating the resulting silicate coated article at a temperature of from about to about 800 F. for a period of 2 minutes to about 24 hours or more.

6. The process as defined in claim 5 wherein the aqueous alkali metal silicate solution is a sodium silicate comprising the equivalent of about 8.9 parts by weight Na,O28.7 parts by weight SiO 62.4 parts by weight H O, said sodium silicate solution being admixed with additional water in an amount to provide a solution having at a minimum about 25 volume percent of the original sodium silicate solution.

7. The process as defined in claim 6 wherein the silicate solution contains from about 1 to about 20 grams per liter of at least one of the members from the group consisting of aqueous soluble borate and aqueous soluble chromate, and mixtures thereof, the concentration of said borate being expressed as NEI1B4O7'10H3O and the concentration of said chromate being expressed as N-a CrO 4H 0.

References Cited UNITED STATES PATENTS 1,451,755 4/1923 Becker 1486.27 1,734,706 11/ 1929 Adler 148'6.16 X 1,923,502 8/1933 Prier 1486.16 3,378,410 4/1968 'Barlett 148-6.27

RAL'PH S. KENDALL, Primary Examiner.

US. Cl. X.R. 

1. A PROCESS FOR PROVIDING A PROTECTIVE SURFACE COATING ON MAGNESIUM WHICH COMPRISES: (A) CONTACTING A MARGNESIUM ARTICLE AT A TEMPERATURE OF FROM ABOUT 70*F. TO ABOUT 220*F. FOR A PERIOD OF FROM ABOUT 1 TO ABOUT 60 MINUTES WITH AN AQUEOUS ALKALINE SOLUTION CONTAINING PHOSPHATE AND CHROMATE ION SOURCE MATERIALS, SAID SOLUTION COMPRISING AN ALKALI METAL CAUSTIC, EXPRESSED AS NAOH, OF FROM ABOUT 5 TO ABOUT 300 GRAMS/LITER, ALKALI METAL PHOSPHATE, EXPRESSED AS TRISODIUM PHOSPAHTE DECHYDRATE, FROM ABOUT 1 TO ABOUT 60 GRAMS/LITER, AND AQUEOUS SOLUBLE CHROMATE ION SOURCE MATERIAL, EXPRESSED AS SODIUM CHROMATE TETRAHYDRATE, FROM ABOUT 1 TO ABOUT 50 GRAMS/LITER, (B) SEPARATING THE SO-TREATED ARTICLE FROM CONTACT WITH SAID SOLUTION, AND (C) HEATING THE SO-TREATED ARTICLE AT A TEMPERATURE OF FROM ABOUT 300 TO ABOUT 800*F. FOR A PERIOD OF FROM ABOUT 2 MINUTES TO ABOUT 8 HOURS OR MORE IN AN OXIDIZING ATMOSPHERE. 